1. Field of the Invention
This invention relates to a negative active material for a lithium secondary battery and a producing method thereof.
2. Description of the Related Art
Lithium secondary battery is generally composed of a positive electrode made of a compound which can accommodate and release lithium ions, a negative electrode made of carbon or the like capable of accommodating and releasing metallic lithium, lithium alloy or lithium ions and an organic electrolyte in which a lithium salt is dissolved in an organic solvent or polymer electrolytes, and shows remarkably high voltage and high energy density.
As a compound to be used as the positive electrode capable of accommodating and releasing lithium ions, lithium cobaltate (LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), spinel type lithium manganese oxide (LiMn.sub.2 O.sub.4), vanadium oxide (V.sub.2 O.sub.5, V.sub.3 O.sub.11) and the like are well known.
As the negative electrode, the just described materials are well known, and other metal oxides capable of accommodating and releasing lithium have also been reported, such as a lithium titanium spinel oxide (LiTi.sub.2 O.sub.4 or Li.sub.4/3 Ti.sub.5/3 O.sub.4) (T. Ohzuku et al., JECS, 142, 1431 (1995)), an oxide mainly composed of tin (Unexamined Japanese Patent Publications (kokai) Nos. 7-12274, 7-201318, 7-288123), an oxide mainly composed of silicon (Unexamined Japanese Patent Publication No. 6-325765, EP 0615296 A1) and the like.
Lithium secondary battery can be classified into two types depending on the combination of positive and negative electrodes when the battery is firstly assembled. One is a combination of a positive electrode which does not contain lithium with a negative electrode comprised of a material that contains metallic lithium and metals (charged state), and the other is a combination of a positive electrode comprised of a material which contains lithium with a negative electrode comprised of a material that does not contain lithium (uncharged state). A typical example of the former case is a combination of a positive electrode comprised of vanadium oxide with a negative electrode comprised of metallic lithium. In this case, the following charge and discharge reactions occur, and discharge is firstly carried out when the battery is used for example, as indicated in the following formulae. ##STR1##
A typical example of the latter case is a combination of a positive electrode comprised of lithium cobalt oxide (LiCoO.sub.2) with a negative electrode comprised of graphite (C). In this case, the charge and discharge reactions progress in accordance with the following formulas, and the charging reaction is firstly carried out after assembly of the battery. ##STR2##
In the metallic lithium battery based on the formulas (1) and (2), short circuit is apt to occur when the charging and discharging are repeated, due to lithium dendritic deposition on the surface of the electrode, and there is a danger of causing abnormal generation of heat and increment of pressure inside the battery, so that it has not been used practically. Substitution of metallic lithium by lithium alloy can hardly change the situation.
In the case of the battery based on the formulas (3) and (4), on the contrary, lithium is inserted in or released from the active material in the form of ions (because of this, this type of battery is particularly called lithium ion secondary battery), so that deposition of metallic lithium can be avoided, its charge and discharge cycle life becomes longer and high safety is ensured. Because of these reasons, lithium ion secondary batteries are now widely put to practical use.
It is desirable that the active materials of positive and negative electrodes have large capacity per unit weight or unit volume and high battery voltage (higher in the case of positive electrode and lower in the case of negative electrode based on Li/Li.sup.+).
As the negative active material, graphite or amorphous carbon is currently put into practical use, and it has been reported that, in the former case, 1 mole at the maximum of lithium is intercalated into 6 moles of carbon atoms (LiC.sub.6) and its theoretical capacity density becomes 372 mAh/g, while a capacity density of 500 to 600 mAh/g is obtained in the latter case, though not fully elucidated theoretically. However, in view of discharge voltage, graphite shows a flat potential of about 0.1 V (based on Li/Li.sup.+), while electric potential of amorphous carbon gradually becomes higher as the discharge progresses and reaches about 0.5 V in average with relatively low battery voltage.
On the other hand, when considered from the viewpoint of energy consumption at the time of producing these carbon materials, they are produced at relatively high temperatures of 2,000.degree. C. or more in the case of graphite and that of 800 to 1,000.degree. C. in the case of amorphous carbon Therefore, the amount of energy consumption is considerably large in both cases, particularly in the case of graphite. Further, from a different view point in terms of the quality control for producing these carbon materials, quality control can be made easily in the case of graphite because of its well developed crystals which make possible precise measurement of the crystal lattice constant by a powder X-ray diffraction as a relatively simple and easy method, while quality control is not easy in the case of amorphous carbon, because clear diffraction peaks cannot be obtained by the powder X-ray diffraction method.
Thus, though they are similar carbon materials, graphite and amorphous carbon have their own respective merits and demerits. Accordingly, when one of them must be selected as the negative active material for lithium ion battery use, the selection is based on their important properties. In any case, the negative electrode using a carbon material is not stable when lithium ions are accommodated in advance which is also a complicated step. Consequently, it is built into the battery generally under an uncharged state (a state of no insertion of lithium ions).
Of the aforementioned lithium titanium spinel type oxides, the material having a composition of Li.sub.4/3 Ti.sub.5/3 O.sub.4 has problems in that its capacity density is about 170 mAh/g which is small when compared with carbon materials, and its discharge potential is about 1.5 V (based on Li/Li.sup.+) which is considerably high. However, since lithium is included at the time of producing, these materials exert an advantage when they are made into charged state at the time of the battery assembly.
On the other hand, it has been reported that the aforementioned tin oxides (Sn(II)O having an .alpha.-PbO.sub.2 structure and Sn(IV)O.sub.2 having a rutile structure) have a capacity density of about 500 mAh/g which is almost the same as that of amorphous carbon (Unexamined Japanese Patent Publication (kokai) No. 7-235293). The publication also describes methods of preparation in the case of SnO.sub.2 for example, it is produced by a method in which a precipitate obtained from a mixture solution of a tin salt and an alkali hydroxide is subjected to iD heat treatment at a temperature of 250.degree. C. or more, preferably 400.degree. C. or more. In this connection, the above report describes that sharp decrease in the discharge capacity density is not found by 10 cycles of the charge discharge cycle, but does not describe test results of more larger charge discharge cycles.
As described in the foregoing, there are a number of negative active materials for use in the lithium secondary battery, but when they are viewed in terms of their characteristics and producing steps, they are not excellent in all points because they have their own respective merits and demerits. In other words, the choices may vary depending on what points should come before all others as evaluation criteria of the negative active material.